Introduction

Acid rain is a short-hand term that covers a set of highly complex and controversial environmental problems. It is a subject in which emotive and political judgements tend to obscure the underlying scientific issues which are fairly easily stated but poorly understood. In this article I shall deal solely with the scientific problems involved in the acidification of surface waters, attempt to establish the facts, describe the present state of knowledge and understanding and discuss what research is needed to provide a firm basis for remedial action.

Although the term acid rain is commonly used to describe all acid deposition from the atmosphere that may cause damage to trees, vegetation, fisheries, buildings, etc., in fact rain (and snow) brings down only about one third of the total acids over the UK, two thirds being deposited in the dry state as gases and small particles. But wet or dry, there is little doubt that acid deposition from the atmosphere poses an ecological threat, especially to aquatic life in streams and lakes on hard rocks and thin soils in southern Scandinavia and in some parts of Scotland and North America. A great deal of research is being undertaken in these three areas but this account is based largely on work in Scandinavia and the United Kingdom where more than 30 research groups from a wide variety of disciplines and institutions are working in a closely integrated and coordinated programme under the author's direction.